How much of the old fireplace can I remove?

Greetings all, my wife and I have moved on to our next remodel project (after doing a complete remodel of our basement to finish it and install a bar and wall mounted TV and all that jazz) and we've decided to update our living room wall that the old fireplace is on. I'll include a bunch of pictures below but the house is a ranch built in the early 60s and has this traditional wood burning fireplace and chimney. We want to modernize the style and redo this entire wall by converting the fireplace to a gas fireplace and doing some built-ins on each side of it (so essentially three "columns". We play to use Norstone or some other drystack/brick veneer in the middle "column".

After doing some research and sending some emails to various chimney/fireplace peoples, I decided that the best way to go was with a fireplace insert and co-linear venting. I have purchased the co-linear vent system so I'm good up the chimney in the venting department. The wife, however, was not to psyched when we realized that about every fireplace insert uses a pretty large surround. For the more modern looks it seems like most things are "clean face" and I now believe that's the way we'd want to go.

So, my research turned to that and we think we are set on using the Napoleon BGD36CF because it has a modern look (doesn't use the brick-look in the firebox but also uses logs, not crystals [we're not huge fans of the crystals]) but a log set. I know I'll have to get a co-axial to co-linear adapter to connect up to my 3" intake/exhaust lines so I'm prepared for that--but what I not up to speed on is how much of the existing fireplace I can safely remove without the chimney falling down

So, why remove anything right? Well, the issue is that these clean-face fireplace (well I guess *all* non-insert fireplaces) are really big! What I'm worried about is that the dimensions of the Napoleon are so big that we're going to have to frame it way out from the existing fireplace and lose some of the real estate we were hoping to gain (from the removal of the hearth). If I can remove more of the current fireplace (especially the bricks on the bottom/underneath the firebox) I think I could safely install the new fireplace back quite a bit.

So, on to the pictures! I have removed all 460 bricks from this sucker (by hand with a cold chisel, I don't know what I was thinking. I could have probably rented a tool to do it in an hour with minimal exertion) and this is what I'm left with. I'm wondering if I can knock out and remove the angled bricks that made of the previous firebox sides (white bricks) and first row bottom bricks (white bricks)? What are the piled bricks there for? Is that to keep the heat contained in the firebox and not needed now that I'll be moving to a zero-clearance? Also, that metal bar that is above the firebox--is that still necessary? If I chip out that mortar there (again so that the new fireplace can be further "back" from the living room) will the chimney collapse? Wood framing I understand and the header in this photo is easy to understand--but how does a chimney work? I mean is that metal bar (only about 3-4" deep) acting as the "header" for the chimney/fireplace opening?

I guess I'm just looking for tips and advice at this point. I feel like I've demo-ed everything I'm comfortable demo-ing until I get more feedback. Up to this point there have been moments where I was pretty confident I could remove bricks and it all turned out to be correct--but I want to understand this old fireplace/chimney construction before I attempt to remove anything else.

The white bricks are "fire brick", design to tolerate the extreme temps of the firebox itself. With a self-contained insert they're not necessary, but if they're what's holding up the steel angle iron supporting the rest of the brick above, you may want to cut in some steel pillars or some other mechanical support for that angle iron if you're removing them completely.

The rubble-brick jumble filler is a common way to isolate the thermal expansion/contraction issues of the hot firebox and the potentially cold/very-cold exterior to keep the masonry from cracking (either the firebox or the exterior), though filler wythes of solid masonry with cavities between them, or even solid masonry are sometimes used. Any non-structural stuff like that isn't really needed for an insert.

What you DO want to do is make it all as air-tight as possible before you button it up, since there is a significant "chimney effect" driving infiltration even for cavities filled with masonry rubble, and if there's room for it, R15 rock wool batting will limit the heat loss out the back side even further, and would still meet code. This appears to be an exterior above-grade wall, and if there is no exterior insulation (usually the case), the fireplace brick represents a big R1-2 hole in your otherwise decently insulated wall, which has both comfort and efficiency ramifications. Check the clearances requirements in the insert spec, but most don't have any problems at all being in contact with non-combustible high-temp insulation like rock wool. Many have no insulation at all, in which case more than half the heat from the insert would be going to the great outdoors, in your configuration, no matter what the efficiency numbers in the spec say.

Use sheet metal for air barriers on the masonry. The tough part to really air seal well is where the Napolean's top vent passes through your home-fabricated sheet metal top-side air barrier, but using automotive muffler-repair putty can narrow the gaps safely. Even though it won't form a permanently-perfect air seal, but even after it develops cracks it's way better than a extra 1-4 square inches of air leak with a 24/365 stack effect pull on it.

You should have a damper and air "ledge" above the fireplace. If you used the original fireplace flue with the insert, you would have to block the damper open so it could not accidently close and prevent drafting.

They would definitely be holding *me* up until a well-seasoned brick mason had taken a thorough look and told me exactly what to do there! Find a good one and offer him $100.00 and a cold beer to come take a look and be sure your chimney does not end up on top of that soft little cat lying there on the living room floor!